Why does the "Map Range" node fail for values above 10,000?
By viewing the source of this node in:

source/blender/compositor/operations/COM_MapRangeOperation.cpp


We see in line 46 this comment:

/* The code below assumes all data is inside range +- this, and that input buffer is single channel */
#define BLENDER_ZMAX 10000.0f


So it is not a bug, this range is defined by the developers for some reason. If the input value is larger than BLENDER_ZMAX then the To Max input is returned, if the input value is less than negative BLENDER_ZMAX then To Min is returned.